You have a sample of returns from Stocks A and B. What is the standard deviation of a portfolio consisting of 70% A and 30% B? Month Stock A Stock B 1 9% 6% 2 21% 8% 3 15% -2% -8% 3% 5 -2% 8% 4 a) 7.9% b) 8.4% c) 7.5% d) 8.2%